Investments - Schedule of Fixed Maturities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year or less	$ 48,012
Due from one to five years	612,267
Due from five to ten years	1,047,056
Due from ten to twenty years	4,027,521
Due after twenty years	7,922,723
Mortgage-backed and asset-backed securities	120,675
Fixed Maturities, Available for Sale, Amortized Cost	13,778,254	$ 13,251,871
Fair Value
Due in one year or less	48,366
Due from one to five years	666,057
Due from five to ten years	1,175,885
Due from ten to twenty years	4,622,554
Due after twenty years	8,802,523
Mortgage-backed and asset-backed securities	124,705
Fixed Maturities, Available for Sale, Fair Value	$ 15,440,090	$ 13,758,024